Electron emission sources operating under basic principles of scanning tunneling microscopes (STM) and electron beam microcolumns based on electron optical elements having microstructures were introduced in 1980. In electron beam microcolumns, microelements are elaborately assembled to minimize optical numerical values, thus forming improved electron columns. Furthermore, due to the microstructures, arrangements of a plurality of electron beam microcolumns are used in serial or parallel multi-electron columns.
The microcolumns are high-aspect-ratio micromechanical structures including microlenses and deflectors. Microlens assemblies constituting the microcolumns are multi-layered silicon chips (with membranes windows for lens electrodes), or silicon membranes which are spaced apart from each other by insulating layers each having a thickness of 100 to 150 μm. The microlens assemblies of the microcolumns include bores having diameters of a few to several hundred micrometers. For optimum performance, the roundness of the bores must be in the nanometer range and the alignment error between elements is required to be within a range of less than 1 μm.
FIG. 1 is a sectional view of a conventional 1 kV microcolumn based on the well-known STM aligned field emission (SAFE), showing a source lens part 1 and an Einzel lens part 3. An electron emission source 5, attached to a positioner of a scanning tunneling microscope (STM) type, emits an electron beam 6 toward a sample plane 25. The electron beam 6 first passes through the source lens 1, which is composed of silicon microlenses and, for example, an axially provided extractor 7 having a diameter of 5 μm, an accelerating electrode 11 having a 100 μm diameter hole, and a limiting aperture 13 with a 2.5 μm diameter hole. Three microlenses are separated by two insulating spacers 9. The insulating spacers 9 are preferably formed of Pyrex, but may be made of an insulating material, such as SD-2 glass made by Hoya. The source lens 1 is mounted on an aluminum base 15 having a deflector 17 which is composed of eight electrodes. Thereafter, the electron beam 6 passes through the Einzel lens 3. The Einzel lens 3 includes silicon microlenses 19 and 23 having 100 to 200 μm diameters. The silicon microlenses 19 and 23 each have at a center thereof a silicon hole unit 21 having a 1 to 2 μm thickness and a 1 mm×1 mm size. Silicon layers are separated by insulating spacers 9 to be spaced from each other at regular intervals. Thereafter, the electron beam 6 enters onto the sample plane 25 to emit a secondary electron. A channeltron detector 27 detects the secondary electron.
To assemble the lens assembly of the conventional microcolumn, the microlenses, made of silicon, and the insulating Pyrex spacers are sequentially layered one after another. Thereafter, the layered lenses and insulating materials are anodic-bonded together. The anodic-bonding is an electrochemical process of coupling glass to metal and semiconductors, as shown in FIGS. 2 and 3. At high-temperature (300-600° C.), sodium and oxygen ions of Na2O in the Pyrex or other glass are activated. When an electric field is formed by applying voltage, supplied from a voltage source 52, between a silicon microlens layer 53 and a glass insulating layer 55, sodium ions in the glass migrate from an interface in a direction shown by the arrow 63. Oxygen anions 61 move toward an induced positive charge 59 in a silicon anode to form chemical bonds.
This process was previously used for single sided bonding only. However, recently, this process has been extended to multilayer bonding. After the first silicon-to-glass bond, another silicon chip or membrane may be bonded to the free surface of the glass by reversing the applied voltage, as shown in FIG. 3. In this case, a second silicon layer 57 is placed on the glass insulating layer 55, while an opposite voltage is applied by the voltage source 52. At this time, the induced positive charge 59 moves the sodium ions in a direction indicated by the arrow 63 such that the oxygen anions 61 form chemical bonds with the second silicon layer 57. To achieve satisfactory multilayer bonding, controlling the temperature, the applied voltage, the bonding time, and, particularly, the surface condition of the layers is very significant.
However, the above-mentioned anodic bonding is executed after a plurality of microlenses and insulating layers are alternately layered. Therefore, while a layered product of the lens assembly of the microcolumn, which requires high accuracy in alignment, is heated to a high temperature and cooled, the layers may become misaligned, and thereby, the accuracy may be deteriorated. Furthermore, for the anodic bonding, an upper electrode is connected into a contact point type using a wire. Accordingly, an excessively long time is required to anodic-bond the lens assembly through the whole area using wire voltage.
In addition to the above-mentioned assembling method, a lens assembly of a microcolumn using laser spot bonding was proposed in Korean Patent Application No. 2001-7003679 (Filed: 22 Mar. 2001), which will be described herein with reference to FIG. 4 and will be quoted in the description of the present invention.
FIG. 4 illustrates a lens assembly 93 of a microcolumn spot bonded by a laser, in which three microlenses 81, 85 and 89 and two insulating layers 77 and 87 are alternately layered.
In the lens assembly 93, the first insulating layer 77 and the second insulating layer 87 each have two extension parts which horizontally protrude outwards from opposite edges of each of the first insulating layer 77 and the second insulating layer 87. That is, the first insulating layer 77 and the second insulating layer 87 have ear parts 79 and 88, respectively. Because a microlens aperture of the microcolumn has a diameter of 2 μm or less, it is imperative that multiple layers of the microcolumn be precisely aligned.
When a laser beam is emitted from a laser, the laser beam substantially passes through the first insulating layer 77 to heat a surface of the second microlens 85. Thus, the first insulating layer and the surface of the microlens are instantaneously welded together. In the same manner, due to the laser beam passing through parts 84 to be welded, a surface of the second insulating layer 87 is instantaneously welded.
In other words, while silicon of the microlenses is melted at a high temperature and, thereafter, recrystallized, an adjacent portion of the insulating layer is heated. At approximately 400 to 500° C., the glass insulating layer begins to flow. At this time, a micro-weld of approximately 100 μm-500 μm in diameter is formed between two layers at the location of a laser spot weld or micro-weld 84.